A low pressure CVD method has been widely used in forming thin films in the course of fabricating semiconductor devices, e.g., IC, LSI or the like. Such process includes deposition of a boron doped silicon film on a substrate.
In the past, diborane (B2H6) used to be utilized in doping boron into a silicon film by using a low pressure CVD apparatus (such as shown in FIG. 1). A boat holding a plurality of wafers vertically stacked is arranged to be loaded into a reaction furnace, and a reaction gas including diborane is introduced into the bottom region of the furnace. The reaction gas is propagated through the upper region, depositing thin films on the substrates. Such arrangement yields rather unfavorable intra-wafer non-uniformities in film thickness and specific resistance of, e.g., about 10 to 20% throughout all the regions from top to bottom.
Furthermore, inter-wafer non-uniformity in specific resistance between the bottom region and the upper region of the CVD apparatus at uniform temperature is even worse to be, e.g., about 30 to 40%. These inter-wafer non-uniformities can be reduced by having a deliberate temperature gradient across the regions or raising a film forming temperature, but raising the growth temperature may poly-crystallize films, which may result in sharp rise in the specific resistance thereof.
As a viable alternative, boron trichloride (BCl3) is employed as a doping gas instead of diborane (B2H6), thereby considerably reducing the intra-wafer non-uniformity in film thickness. Referring to FIG. 2, there is shown a comparison of the intra-wafer non-uniformity of thickness of boron doped polysilicon films, wherein B2H6 gas and BCl3 gas are varied as boron sources, respectively. (in both cases monosilane (SiH4) gas is fixed as a silicon source) The y-axis represents intra-wafer non-uniformity in film thickness of the boron doped polysilicon in percentage and the x-axis represents the location of the wafer in the boat, in terms of the slots (see FIG. 1).
As can be clearly seen from FIG. 2, boron trichloride BCl3 provides a better intra-wafer uniformity in the film thickness than the diborane B2H6. Even in the case of using BCl3 as a doping gas, however, uniformity in the intra-wafer uniformity in the film thickness still ranges from about 5 to 6% in the bottom region (i.e., the region ranging from slot Nos. 11 to 36 in FIG. 1), which is still inadequate for use in a semiconductor device. Accordingly, there has been a continuous search for a way to improve the intra-wafer uniformity of the film thickness.
The inter-wafer uniformity in the specific resistance of the boron doped polysilicon film is considerably improved by replacing diborane B2H6 with boron trichloride BCl3 as a source of boron. For instance, boron doped polysilicon formed by using monosilane SiH4 and boron trichloride BCl3 under the condition of a partial pressure of SiH4 at about 63.4 Pa and a partial pressure of BCl3 at about 3.2 Pa, wherein the flat film forming temperature ranges from about 400 to 420° C., yields the inter-wafer non-uniformity of the specific resistance thereof amounting to about 10%, which is also inadequate for use in a semiconductor device requiring non-uniformity to be less than 3%.
One of the major factors attributing to the inter-wafer non-uniformity in the specific resistance is the spatial non-uniformity in the partial pressure of boron trichloride BCl3 in the reaction furnace. More specifically, portions of boron trichloride BCl3 and monosilane SiH4 that are respectively supplied into the reaction furnace are spent in forming thin films on the wafers and the rest is exhausted out of the reaction furnace, during which boron trichloride BCl3 is consumed at a different rate from that of monosilane SiH4, thereby resulting in a non-uniform partial pressure of boron trichloride BCl3 within the reaction furnace. Accordingly, it is of a practical concern to provide a condition for obtaining uniform inter-wafer specific resistance, e.g., irrespective of partial pressure of boron trichloride BCl3.